Mac Botnet Shrinks As Defensive Efforts Take Hold

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The number of Macs infected with the Flashback malware has fallen by more than half, an indication of the success of efforts to destroy the largest network to date of compromised Apple computers, a security vendor said.

Since reaching its peak last week of 600,000 Macs, the botnet has been getting smaller, comprising 270,000 systems as of Thursday, Cupertino, Calif.-based Symantec said. The majority of infected systems were in the United States and Canada, with fewer found in the U.K., Europe and Australia.

The publicity surrounding the botnet, which accounted for more than 1 percent of all Macs in use, has contributed to the erosion, Liam O Murchu, manager of operations for the Symantec Security Response team, said. Once made aware of the infection, Mac users have been checking their systems and using removal tools, some of which security vendors are providing for free. In addition, Apple has been working with Internet service providers to take down command-and-control servers, making it difficult for the malicious network to grow.

The botnet is expected to fade as the intensity of the publicity increases the chances of the attackers getting caught, Murchu said. Because the same criminals have been targeting Macs with Flashback since September 2011, they are likely to return with another variant of the malware, which is capable of stealing passwords used in online banking or other websites. "They put a bit of work in this already, so it is conceivable that they would just move on to create a new version and start infecting people again," Murchu said.

The attackers have had so much success in the latest campaign, because they have targeted a vulnerability in the Mac that makes it possible to install the malware when someone visits a malicious website. The flaw is within the Java application platform, and the doorway is the Web browser. Mac users were probably directed to the malicious site after clicking on a link secretly embedded on a legitimate website, Murchu said.